1. Field of the Invention
This invention relates to an optical head device for applying a light beam to the information recording surface of an information carrier and effecting the detection or recording of information, and in particular to an optical head device which is compact, light in weight and suitable for mass production.
2. Description of the Prior Art
According to the prior art, an optical head device is constructed as shown, for example, in FIG. 1 of the accompanying drawings. A divergent light beam emitted from a laser light source 1 enters a collimator lens 2 and becomes a parallel light beam, and enters a polarizing beam splitter 3. The polarizing beam splitter 3 has a characteristic of transmitting therethrough about 100% of a linearly polarized light having a plane of vibration in a particular direction and reflecting about 100% of a linearly polarized light having a plane of vibration in a direction orthogonal to said particular direction. The linearly polarized light transmitted through this polarizing beam splitter 3 passes through a quarter wavelength plate 4 and becomes a circularly polarized light beam, and is condensed on an information recording surface 7 provided on the substrate 6 of an information carrier, by an objective lens 5, and forms thereon a spot having a spot diameter of about 1 .mu.m. Also, the light beam reflected by this information recording surface 7 passes through the objective lens 5 and becomes a parallel light beam, and passes through the quarter wavelength plate 4 and becomes a linearly polarized light beam having a plane of vibration orthogonal to that during incidence, and again enters the polarizing beam splitter 3. The polarizing beam splitter 3 acts as a beam splitter due to its characteristic as described above, and reflects the light reflected from the information recording surface 7 and separates it from the incident light, and directs it as a convergent light beam to a photodetector 10 through a sensor lens 8 and a cylindrical lens 9.
To record information by the use of such an optical head device, the laser light source 1 is driven in accordance with an information signal and the light incident on to the information recording surface 7 is modulated, thereby effecting the recording. Also, to detect the information, an unmodulated light beam is applied to the information recording surface 7 on which the information is recorded by means of concavo-convex pits or variations in reflectance, and the reflected light subjected to modulation by this recorded information is detected by the photodetector 10, whereby the information is reproduced.
Also, where the information magnetically recorded on the information recording surface is to be read out by the use of the magneto-optical effect, a polarizing beam splitter 11 as shown in FIG. 2 of accompanying drawings is used instead of the aforedescribed polarizing beam splitter 3 and the quarter wavelength plate 4. The polarizing beam splitter 11 comprises rectangular prisms 12 and 13 and a reflecting film 14 which is formed so as to reflect or transmit P-polarized light and S-polarized light at a suitable percentage. For example, as regards the P-polarized light beam 15 entering the polarizing beam splitter 11, 30% of the energy thereof is reflected by the reflecting film 14 and becomes a light beam 17, and a light beam 16 of remaining 70% is transmitted and is condensed on the recording surface of the an opto-magnetic recording medium through an objective lens (not shown). A reflected light beam 18 having its plane of polarization rotated (Kerr rotation) in conformity with the information on the recording surface again enters the polarizing beam splitter 11. The component modulated by the Kerr rotation is S-polarized light and is reflected by about 100% by the reflecting film 14. On the other hand, 70% of the P-polarized component of the reflected light 18 is transmitted through the reflecting film 14 and only the remaining 30% is reflected thereby and is directed to the photodetector with said S-polarized component. By thus increasing the modulated component (S-polarized light) relative to the original-polarization component (P-polarized light) of the incident light, the angle of Kerr rotation is apparently increased and signal read-out with a high S/N ratio becomes possible.
Also, in the optical head device, autofocusing for focusing the light from the light source consistently on the information recording surface is effected to record information very densely on the information recording surface and detect the highly densely recorded information. The device shown in FIG. 1 is an example using the conventional astigmatism method. The cylindrical lens 9 causes the reflected light to create astigmatism. Also, the photodetector 10 has its light-receiving surface divided into four parts and is so arranged that a circular light intensity distribution is created on the photodetector 10 when the information recording surface 7 is in the in-focus position of the objective lens 5, that is, when the light spot is stopped down to a predetermined size of the order of 1 .mu.m on the information recording surface 7. As a result, when the information recording surface 7 is moved back and forth from the focus position of the objective lens 5, the light distribution on the photodetector 10 changes into an elliptical shape having its major axes orthogonal to each other. Accordingly, by comparing the outputs of the light-receiving surfaces of the photodetector 10 and detecting the change in the light distribution, there is obtained a focus error signal, and the objective lens 5 is moved along the direction of the optical axis by an actuator, not shown, in accordance with the focus error signal, whereby auto-focusing is accomplished.
However, the conventional optical head device as described above requires a sensor lens or the like, and this has hindered achievement of compactness and low cost in the device. Also, to obtain a good signal from the photodetector, it is necessary to place an optical element such as the sensor lens or the like at an accurate position and angle relative to the optical axis of the detected light, and this has complicated the assembly and adjustment. Further, both of the aforedescribed polarizing beam splitters 3 and 11 are made by joining the opposite surfaces of two prisms together and therefore, complicated working and alignment adjustment are necessary, and this has led to the disadvantage that it is difficult to reduce the cost of the device. Also, these polarizing beam splitters are substantially cubic in shape, and this has been a factor which impedes the thinning of the device when such polarizing beam splitters are used in an optical head device or the like.
An optical head device which does not require the above-mentioned sensor lens or the like is proposed in Japanese Laid-Open Application No. 8145/1984. In this device, the joined surface of a prism type beam splitter as described above is curved and endowed with a condensing action as a concave mirror. In this device, however, when making the beam splitter, prisms each having a convex surface or a concave surface must be individually polished and then assembled together, and this has not been suitable for mass production. Further, a reflecting film of polarized light dependency is provided on the joined surface of the beam splitter, but where the joined surface is made into a spherical surface or a cylindrical surface, the angle of incidence of light onto this reflecting film differs from location to location and the polarizing characteristic varies. Accordingly, strict polarized light dependency cannot be expected from such a construction.
On the other hand, examples using a volume type diffraction grating in place of the aforementioned prism type beam splitter are described in Japanese Laid-Open Patent Application No. 155508/1982 and U.S. Pat. No. 3,622,220. The construction of an optical head device using such a volume type diffraction grating is shown in FIG. 3 of the accompanying drawings. In FIG. 3, a parallel light beam 22 emitted from a light source unit 21 including a laser light source and a collimator lens enters a volume type diffraction grating 23 placed at an angle of 45.degree. relative to said light beam. The light beam 22 is S-polarized light having a plane or vibration in a direction perpendicular to the plane of the drawing sheet. The volume type diffraction grating 23 has a pitch substantially equal to .lambda./1.414, where .lambda. is the wavelength of the incident light, and deflects the light beam 22 at an angle of diffraction of 90.degree.. At this time, the diffraction efficiency for S-polarized light is approximately 100% and the diffraction efficiency for P-polarized light is approximately 0%. Accordingly, the light beam 22 is almost diffracted, and is transmitted through a quarter wavelength plate 24 and becomes a circularly polarized light beam, and forms a spot on the information recording surface 27 of an optical disc 26 with the aid of an objective lens 25.
Information is recorded on the information recording surface 27 by a variation in reflectance or the like, and the reflected light therefrom is subjected to light amount modulation in accordance with said information. This reflected light is transmitted through the lens 25 and the quarter wavelength plate 24 and becomes P-polarized light and enters a photodetector 28 without being diffracted by the diffraction grating 23, and said information is read.
However, since in the construction shown in FIG. 3, the angle of diffraction of the volume type diffraction grating is set to approximately 90.degree., this construction cannot be used to read the information from an opto-magnetic recording medium. The reason will hereinafter be explained. In FIG. 3, where information is magnetically recorded on the information recording surface 27, the quarter wavelength plate 24 is removed from the optical path. The S-polarized light beam entering from the diffraction grating is reflected as a light beam whose plane of polarization has been rotated (Kerr rotation) by the same amount in the reverse direction, depending on whether the direction of magnetization of the information recording surface 27 is upward or downward. This angle of Kerr rotation is an angle as small as the order of 1.degree. and thus, most of the reflected light is an S component and includes small P components different in direction and equal in size. Since the diffraction grating 23 has an angle of diffraction of 90.degree., the S component of said reflected light is almost diffracted toward the light source unit 21. On the other hand, only the P component created by the Kerr rotation is transmitted intact and enters the photodetector 28. As previously described, the P components corresponding to the direction of magnetization are just opposite in direction and equal in size and therefore, even if an analyzer is provided, at any azimuth angle, in front of the photodetector 28, the light amount transmitted therethrough is the same irrespective of the direction of magnetization and the information cannot be read. Thus, a beam splitter having an appropriate polarizing characteristic is indispensable for the reading of the information on an opto-magnetic recording medium, but in the construction according to the prior art, such a beam splitter could not be realized.
Also, in the conventional optical head device, the incident light beam and the diffracted light beam form an angle of 90.degree. with each other within the volume type diffraction grating. Therefore, in the usual form, the incident light is totally reflected on the surface of the diffraction grating and, although not shown in FIG. 3, but in reality, as shown in the aforementioned U.S. Patent, a construction in which a volume type diffraction grating is disposed between rectangular prisms must be adopted and it has been impossible to sufficiently make the best use of the features such as compactness and light weight inherent in a diffraction grating.
FIG. 4 of the accompanying drawings is a schematic view showing still another example of the conventional optical head device proposed in Japanese Laid-Open Patent Application No. 64335/1982. In FIG. 4, a light emitted from a semiconductor laser 31 is collimated by a collimator lens 32 and condensed on an information recording surface 37 on a substrate 36 by an objective lens 35. For the reading of the signal on the information recording surface 37, use is made of the so-called SCOOP system in which the variation in the output of the semiconductor laser 31 caused by a variation in the amount of the return light reflected by the information recording surface 37 and returning to the semicondcutor laser 31 through the same optical path so that during the incidence is detected by a monitoring sensor 45. Also, a sensor lens 41 having a focal length f occupies a part of the pupil plane of the objective lens 35, and part 46 of the reflected light from the information recording surface is made into a convergent light beam 47 by the sensor lens 41 and enters a two-division photodetector 42. The convergent light beam 47 moves to the left and right on the photodetector 42 when the information recording surface 37 has become far from or near to the focus position of the objective lens 35. So, a focus error signal is obtained by subtracting the outputs of the divided light-receiving surfaces by means of a subtractor 43. A focusing actuator 44 is driven by this focus error signal and auto-focus is effected so as to ensure the incident light to be focused onto the information recording surface 37.
In the device shown in FIG. 4, the off-axis light beam of the objective lens is taken out and therefore, the detected light beam greatly acts on the focus deviation and thus, focus error detection of high sensitivity can be accomplished. On the other hand, however, the light beam entering the objective lens 35 from the semiconductor laser 31 is eclipsed and deformed by the sensor lens 41, and this has led to a disadvantage that the spot on the information recording surface 37 becomes large.
FIG. 5 of the accompanying drawings is a schematic view showing yet still another example of the conventional optical head device used for the reading of the information from an opto-magnetic recording medium. In FIG. 5, a light beam emitted from a semiconductor laser 51 (hereinafter simply referred to as LD) is converted into a parallel light beam by a collimator lens 52. The parallel light beam then passes through a beam splitter 53 and is condensed into a spot of a diameter of about .phi.1 .mu.m on a recording medium 55 by an objective lens 54. The light beam reflected from the recording medium 55 has its plane of polarization subjected to modulation by the Kerr effect and the Faraday effect and again passes through the objective lens 54, and is separated from the incident light beam by the beam splitter 53. The separated light beam is partly reflected by a second beam splitter 56 and passes through a lens system 57 into an optical sensor 58. The lens system 57 is comprised of conventional systems, for example, an astigmatism system, a knife edge system and a four prism system, and the information of the spacing between the recording medium 55 and the objective lens 54, i.e., AF error signal, is obtained therefrom. Also, the deviation with respect to the information track, i.e., AT error signal, is obtained by the conventional push-pull method or the like. These error signals are fed back to the driving system (generally called the actuator), not shown, of the objective lens and tracking is accurately effected at an accurate focus position, whereby detection or recording of the signals is accomplished.
The remaining light beam passing through the second beam splitter 56 passes through a half wavelength plate 59 and is divided into two directions by a polarizing beam splitter 60. If the half wavelength plate 59 is disposed with its optical crystal axis inclined by 22.5.degree. with respect to the polarization axis of the incident light beam, the light amounts divided into two by a polarizing beam splitter 60 are equal to each other and such half wavelength plate becomes equivalent to a polarizing plate disposed with the respective light beams being endowed with transmission axes of 45.degree. and -45.degree.. The light beams divided into two are converged on signal detecting sensors 63 and 64 by sensor converging lenses 61 and 62, respectively, and the electrical signals from the signal detecting sensors 63 and 64 are differentiated (differential detection), whereby detection of the information signal on the recording medium 55 can be accomplished.
However, the optical head device shown in FIG. 5 can detect signals of good S/N ratio by differential detection while, on the other hand, it requires a number of parts and thus has been disadvantageous in making the device compact and low in cost.